Yep, only one. And if you try to create another object or click on the wrong tool, the object you were working on becomes "stamped" into the background. In other words, it becomes part of the 2d background and is lost as a 3d object. The only way to avoid that is to save the 3d object first. Do you know what you have to save it as? You have to save it as a brush! Then you can get it back by selecting that brush and clicking in the document window. But when you do that, the new 3d object you created becomes the background and is lost...